Earth expansion/growth within the core/mantle will fracture and fragment this crust to form a network of rift zones surrounding the ancient crusts. These rift zones represent low-lying areas and hence will form the loci for the continental seas, plus sediments eroded from the exposed lands will accumulate in these low-lying areas. Continue this process through time and the rifts will continue to stretch and sediments will continue to erode from the exposed lands (note; sediments will not accumulate on high ground - only low ground - basins. The presence of sediments automatically implies that these were originally low-lying sedimentary basins).

Moving forward in time to the Permian Period (~250 million years ago) the ability for the continental crust to rift and stretch was finally exceeded and we had rupturing of the continental crust to form the modern continents plus opening to form the modern oceans. The rest is history for you and me.

In order to determine the primordial Earth size, we have the existing Earth to work from, plus the distribution of existing rocks (note, the granite/greenstones represent the most ancient crusts but do not necessarily occur under all crusts). By working back in time we can progressively remove the sediments from each of the sedimentary basins and return these to the exposed lands. In doing this we also progressively close each of the rifts/basins and reduce the radius of the Earth.

Continue this process back long enough in time and we are simply left with the most ancient granite/greenstone plus/minus sedimentary crusts - all else have been returned to where they came from. Coincidently, but as expected each of these ancient crusts assembled together precisely to form a primordial Earth at approximately 1700km radius (in exactly the same way as we close off the modern oceans to form the Pangaean assemblage).

The 1700km is dictated by the surface area of remnant ancient crusts existing at that time. we then simply married the 2 facts (surface area of remnant ancient crusts, and ages of these crusts) and concluded that the radius of the primordial Earth at approximately 4000 million years ago was approximately 1700km.

The fact that the vast majority of crustal rocks existing at that time were of magmatic and volcanic origin i.e. they were molten when they were originally formed and subsequently cooled and solidified to form the crust, suggested to me that, prior to approximately 4000 million years ago the Earth must have been molten - because there was so much of the stuff being intruded or extruded at the same time). If the primordial Earth were molten prior to 4000 million years ago then it goes a long way towards explaining why we do not have age dates older than this time. This time - which I call the pre-Archaean is indeterminate, i.e. you cannot assign an arbitrary time of 5,000 million years as you suggest. This time can be 10,000 million years or more, hence our start point could be much further back in time than previously implied. That is why I choose to start at my known point in time, 4000 million years ago, at 1700km radius.

The confusion regarding molten/melting is clearly justified. However, to go from a plasma state to a solid state we must pass through a molten state, hence my stance that the pre-Archaean (pre-4000 million year old Earth) was already molten. Moving forward in time the Earth then cooled and solidified. This is the mistake that the Pangea Theorists make, in that they imply that solid planetisimals accumulated, then magically melted, then magically cooled again. This is not what I am implying at all.